Compact image sensor devices used in camera modules are well known in the art. One or more lenses are used to focus light onto the image sensor surface. Lenses can also be used for functions such as zoom in, zoom out, focus adjustment, wide-angle and many other optical effects. However, lenses can also introduce different types of optical aberrations. One optical aberration in particular is the Petzval Field Curvature, in which a flat object normal to the optical axis cannot be brought into focus onto a flat image sensor. One or more additional field curvature correction elements would need to be introduced in the lens stack to counteract the field curvature of an optical system.
FIG. 1 illustrates a conventional image sensor system 1 without any focus plane correction optical elements. The image sensor system 1 include a lens 2 that focuses light onto an image sensor 3. The focus plane 4 of lens 2 is actually not perfectly planar, but rather is slightly curved, so that when light in the center of the system 1 is properly focused onto the image sensor 3, light away from the center axis of the lens is focused slightly above the image sensor 3. This aberration degrades the quality of the image being captured by the image sensor.
One solution to correct for this type of aberration is to add one or more field curvature correction elements to the lens stack, so that the focus plane 4 is planar and located at the planar surface of the image sensor 3. One example of a curvature correction element is a field flattener lens 5, as shown in FIG. 2. However, using curvature correction elements increases the complexity of the optical system, increases the optical system height, introduces an additional point of failure (e.g. the aberration correction element can be misaligned, have uneven surface, etc.), can introduce additional distortion, can decrease brightness, and possibly results in other negative effects.
Another solution is to reconfigure the image sensor to better match a curved focus plane so that the number of optical elements in the lens stack can be reduced. For example, U.S. Pat. No. 6,556,349 discloses using a transparent substrate over the image sensor with varied sizes or heights of the micro lenses above each photo detector. U.S. Pat. Nos. 6,486,917 and 7,923,793 disclose bending and bonding the image sensor to a concave surface. These solutions, however, introduce other problems such as crosstalk, structure height, reduced thermal stress tolerance, special fabrication tooling requirements, reduced three-dimensional integrated circuit (3D IC) capabilities, etc.